This invention relates to an internally plasticized poly(vinyl chloride)-containing block copolymer and a method of producing same. More specifically, it relates to a novel ABA type block copolymer wherein said A groups are formed from a vinyl monomer such as vinyl chloride and said B group is poly(ethylene oxide). This invention also relates to a process which employs macrobiradicals to initiate the polymerization of vinyl chloride.
It is well known that the flexibility of poly (vinylchloride) (PVC) may be significantly increased by the addition of an external plasticizer. This type of additive is effective at concentrations of 30 parts or more per 100 parts by weight of polymer, and functions by simply lubricating the polymer matrix. With this modification, normally rigid resins such as PVC may be used in sheet molding applications such as automotive seat covers. However, the typical external plasticizer tends to migrate to the surface of the material and then to evaporate, and this tendency generates a variety of problems.
In a product made from an externally plasticized poly(vinyl chloride) (PVC), the plasticizer migration problem is generally significant only if the plasticizer concentration exceeds 20 parts by weight per 100 parts of the resin (hereinafter "phr"). Unfortunately, 30 phr or more are usually required to provide adequate flexibility. Therefore, although the ultimate goal is to eliminate the external plasticizer, it would be a significant advance if the required plasticizer concentration could be reduced to 20 phr or less.
In view of this, efforts have been directed to incorporating a flexible polymeric group(s) having a glass transition temperature typically below minus 10.degree.C. directly into the backbone of polymer molecules and thereby increasing their flexibility. Since this flexible "internal plasticizer" is part of the molecular structure, there is no plasticizer migration problem. The immediate success of these internally plasticized polymers, which are in reality copolymers, has led to the development of a growing art directed at the creation of tailored copolymeric materials specifically designed for particular applications.
The prior art teaches the copolymerization of vinyl chloride with many other vinyl monomers, such as vinylidene chloride, vinyl esters, vinyl acetates and acrylonitrile. The resulting copolymers do have a lower softening temperature but, unfortunately, the brittle point of these materials is not much lower than that of the basic poly(vinyl chloride) resin. Therefore, the low temperature properties were not significantly improved by the prior art techniques.
The prior art also teaches the use of certain macroazonitriles in an addition polymerization reaction with vinyl chloride to produce poly(vinyl chloride)-containing block copolymers. Furukawa et al disclosed this technique in Angewandte Makromolekulare Chemie 1 (1967) 92-104 (Nr. 10). However, Furukawa et al used a bulky isocyanate structure and low molecular weight poly(propylene oxide) to build his macroazonitrile. The resulting low molecular weight copolymers contained short, relatively inflexible poly(propylene oxide) segments connected to the stiff poly(vinyl chloride) segments through long, stiff, urethane-containing linkages. Such copolymers would offer little or no improvement over unplasticized PVC.